The present invention relates to frame buffer memory systems for raster displays, and more particularly to an apparatus for facilitating rapid scrolling of raster displays in either vertical or horizontal directions.
Raster scan frame buffer displays have become increasingly popular as the price of semiconductor memory has decreased. The image to be displayed is represented in a large memory that saves a digital representation of the intensity and/or color or each picture element, or pixel, on the screen. By properly recording the data in the memory an arbitrary image can be displayed, making the display hardware insensitive to image content. The frame buffer memory is equipped with hardware to generate a video signal to refresh the display and with a memory port to allow a host computer or display processor to change the frame buffer memory in order to change the image being displayed.
Interactive graphics applications require rapid changes to the frame buffer memory. Although the speed of the host display processor is clearly important to high performance, so also are the properties of the memory system, particularly update bandwidth, the rate at which the host processor or data processor may access the frame buffer memory. For a given memory technology the implicit geometry of frame buffer memory access can affect this rate.
The process of scrolling an image, or a portion of an image on a screen involves reading pixel data from one area of a frame buffer memory and writing it to another area. In the prior art, frame buffer memories have been arranged such that groups of pixels along scan lines are stored at sequentially addressed memory locations. Scrolling speed has been improved by providing first in, first out (FIFO) buffers for storing several words of pixel data rapidly read from such sequential memory addresses, with the low bits of the addresses being rapidly incremented by a counter rather than by the host display controller. The data stored in the FIFO buffer is then written back into memory at the new address sequence also utilizing the counter to rapidly increment the address. While this approach improves scrolling speed, further improvement in scrolling speed is desirable.